Coffee Roasting
 Coffee roasting is a chemical process by which aromatics, acids, and other flavor components are either
created, balanced, or altered in a way that should augment
the flavor, acidity, aftertaste and body of the coffee as
desired by the roaster.
Roasting Coffee Beans
The first stage is endothermic. The
green beans are slowly dried to become a yellow color and
the beans begin to smell like toast or popcorn.
The second step, often called the first
crack, occurs at approximately 205 °C (400 °F) in which
the bean doubles in size, becomes a light brown color, and
experiences a weight loss of approximately 5%. The
corresponding Agtron number for this color is between 95-90
(Davids, 68-69).
In the next step the temperature rises from
205 °C to approximately 220 °C, the color changes from light
brown to medium brown (Agtron # 60-50), and a weight loss
of approximately 13% occurs (Davids, 68-69). The resulting
chemical process is called pyrolysis and is characterized
by a change in the chemical composition of the bean as well
as a release of CO2.
The second step is followed by a short endothermic
period which is followed by another exothermic step called
the second crack. This second pyrolysis occurs between
225-230°C, and the roast color is defined as medium-dark
brown (Agtron #50-45) (Davids, 68-69). The second
pop is much quicker sounding and the beans take on an oily
sheen.
 Espresso
potential is maximized in roasting when you maximize the
sweetness and aroma of the coffee while minimizing the bitterness
and acidity. Most people focus on the latter and therefore
roast extremely dark, yet without sweetness and aroma the
espresso will never be palatable. This explains the
unpopularity of straight espresso and the popularity of
espresso based drinks where either milk or other flavors
are used to replace the sweetness that was lost by roasting
darkly.
From 170-200°C the sugars in
coffee begin to caramelize. From tasting pure sugar
versus its caramelized component it is evident that uncaramelized
sugar is much sweeter. The dark color of coffee is
directly related to the caramelization of the sucrose in
coffee. Therefore, to maximize sweetness you want
to minimize the carmelization of sucrose, yet you do not
want to roast too lightly or bitter tasting compounds will
not thermally degrade. Stop the roast somewhere between
the end of the first crack and less than half way through
the second crack. Do not roast well into or past the
second crack. We recommend a roasting chamber temperature
somewhere between 205-215°C. Realizing the danger
of the following suggestion we might recommend a color similar
to the one below. Note: All monitors, computers, and
internet browsers will display the color slightly different.
This is only a recommendation to point out that this color
is preferable to the almost black color you will frequently
observe for espresso. To get a better idea of roasting
colors order the Agtron roasting classification kit from
the SCAA.
One way to ensure the same roast degree
is achieved each time it is necessary to purchase a set
of Agtron tiles from the SCAA or purchase an Agtron
roast analyzer.
Videos of sample roasting: Medium (4.2 Mb), or Small (1.32
Mb).
Related
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and chemical reactions by Carl Staub.
Espresso
Aroma
Espresso
Blends
Espresso
Roasting
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Chemistry- Aroma
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Chemistry- Acidity
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Chemistry- Bitterness
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